1. Field of the Invention
The present invention relates to image communication apparatus which has at least one of communication control unit for receiving and transmitting image data of different resolutions on at least one communication line and which also has an image memory for storing image data to be transmitted.
2. Description of the Prior Art
The CCITT (International Telegraph and Telephone Consulting Committee of the International Telecommunications Union) recommends several standard facsimile apparatuses, including: Group 1 (Gl), Group 2 (G2), Group 3 (G3) and Group 4 (G4). Facsimile apparatus of the G1, G2 and G3 type use public analog telephone lines, whereas facsimile apparatus of the G4 type mainly use digital communication lines.
Picture element density or image resolution is different between G3 and G4 types of facsimile apparatus. The standard picture element density used in the G3 type is 8 pels. (picture elements)/mm (millimeter) in the main scanning direction, and 3.85 pels/mm sub scanning direction. On the other hand, the picture element density of the G4 type facsimile apparatus is 200 pels/inch (7.87 pels/mm) in both the main and sub scanning directions.
A standard encoding/decoding method known as the MH (Modified Huffman) method, is generally used for encoding and decoding image signals in a G3 type facsimile apparatus whereas a method known as MMR (Modified Modified READ) is generally used for encoding and decoding image signals in a G4 type facsimile apparatus.
Applicants have previously proposed facsimile apparatus which has both G3 and G4 facilities. FIG. 2 shows the block diagram of such dual function facsimile apparatus. As shown in FIG. 2 there is provided an image reading unit 1 for reading an original image to be transmitted. The reading unit 1 reads the original image and converts the original image to an image signal. There is also provided an image printing unit 2, for example a thermal printer, which prints an image corresponding to a received image signal.
The facsimile apparatus of FIG. 2 also includes a digital communication control unit 8. This control unit connects the apparatus with digital communication lines; and it controls communications in the manner of a G4 facsimile apparatus with encoding/decoding according to the MMR method. The apparatus of FIG. 2 also includes an analog communication control unit 9 which connects the apparatus with an analog communication line, for example a telephone line; and carries out modulation/demodulation and encoding/decoding according to the MH method.
There is also provided a reading unit 1 and a printing unit 2 which are connected to an image memory 5 via, respectively, an encoding circuit 3 and a decoding circuit 4. The G4 communication control unit 8 and the G3 communication control unit 9 are connected via the encoding/decoding circuits 6 and 7, respectively, to the image memory 5.
The image memory 5 is a random access memory device, for example a semiconductor device or a hard-disc apparatus.
The above described units are controlled by a control unit (not shown) such as a microprocessor or the like.
The apparatus shown in FIG. 2 transmits an image signal, read from an original by the reading unit 1, to an analog or digital communication line via the analog or digital communication control unit 8 or 9. The signal from the reading unit 1 is communicated through the path E or C in the memory 5 shown in FIG. 2. The apparatus of FIG. 2 also supplies the recording Unit 2 with an image signal from an analog or digital communication line. The image signal is received by the analog or digital communication unit 8 or 9, and is supplied through the memory 5. The signal to the printing unit is communicated through the path A or F in the memory 5 shown in FIG. 2. In addition, image signals may be transmitted directly from the reading unit 1 to the printing unit 2 through a path D in the memory 5; and image signals may be transmitted between an analog and a digital communication line through the communication control units 8 and 9 and a path B in the memory 5.
Generally, in the above-described transfer of image data, in order to make efficient use of the image memory 5, the memory is arranged to store the image data as encoded by the encoding circuit 3 and the encoding/decoding circuits 6 and 7.
It does not matter which one of the above-mentioned coding processes (i.e. MH or MMR) is used by the encoding circuit 3, the decoding circuit 4 and the encoding/decoding circuit 6 and 7.
The differences between the G3 and G4 type communication control units involve not only their respective encoding methods, but also the image resolution. For example, the image resolution of the reading unit 1 and the printing unit 2 in both the main and the sub-scanning directions is generally 400 ppi (picture elements per inch), which is available for both the G3 and G4 type information processing. In this case, the image data obtained by the reading unit 1 may be sent to the printing unit 2 through the path D in the image memory 5, and the image data is copied. Here the size of the read image is the same as the size of the printed or reproduced image.
However, the image resolution of the G3 type image data from the analog communication line which is received via the communication control unit 9 is, as above mentioned, 8 pel/mm in the main scanning direction and 3.85 pel/mm in the sub scanning direction (i.e. 8.times.3.85 pel/mm). Therefore the image resolution of the printing unit 2 is larger than that of the G3 type image data, so that the G3 image data obtained via the path A should be reduced in both the main and sub scanning directions. Similarly, when a relaying operation from the analog line to the digital line is carried out through the path B, the image resolution of the G3 image data should be converted to compensate for the difference between the resolution of the G3 image data and that of the G4 image data.
When the read image data is to be transmitted via the communication control unit 8 to the G4 digital data line, the image resolution of the G4 image data must be half of that of the reading unit 1; and accordingly the resolution of the image data read by the reading unit 1 must be halved. Also, when the image data read by the reading unit 1 is to be transmitted via the communication control unit 9 to the G3 analog date line, the resolution of the image data read by the reading units should be also converted.
As can be appreciated from the foregoing, because the system of FIG. 2 inputs and outputs image data or different image resolution, it is necessary to provide image resolution converting circuits at several positions shown by P1, P2 and P3, in order to communicate between all the combinations of apparatus which operate with different image resolutions. Therefore, the structure of the above-described system is complicated and costly.